1. Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine.
2. Description of the Related Art
In present-day automobiles, a three-way catalytic is installed in an exhaust passage of an internal combustion engine to purify the exhaust gas. As is well known, the three-way catalytic exhibits a high purification ratio for any one of HC, CO, and NOx in the neighborhood of the stoichiometric air-fuel ratio, so air-fuel ratio control is performed with oxygen concentration sensors provided upstream and downstream of the catalytic so that the atmosphere of the catalytic is maintained in the neighborhood of the stoichiometric air-fuel ratio.
The air-fuel ratio upstream of a catalytic converter is controlled to a value in the neighborhood of the stoichiometric air-fuel ratio using the oxygen concentration sensor provided upstream of the catalytic converter. However, the oxygen concentration sensor provided upstream of the catalytic converter is exposed to a high-temperature exhaust gas, causing output fluctuations, so the oxygen concentration sensor provided downstream of the catalytic converter is used to correct errors.
The amplitude of fluctuations in the output from the oxygen concentration sensor provided downstream of,the catalytic converter is small because the atmosphere exhaust gas temperature is low. Moreover, the exhaust gas has been sufficiently mixed and the air-fuel ratio is close to its equilibrium state in a region downstream of the catalytic, so it is possible to detect a purification state of the catalytic accurately and stably.
Therefore, as long as the air-fuel ratio downstream of the catalytic converter is maintained in the neighborhood of the stoichiometric air-fuel ratio, it is possible to maintain a good purification state of the catalytic.
Thus, there is already well known a method for adjusting the air-fuel ratio upstream of the catalytic converter through proportional calculation and integral calculation, which are performed on the basis of a difference in the air-fuel ratio downstream of the catalytic converter from the stoichiometric air-fuel ratio, using a control system for maintaining the air-fuel ratio downstream of the catalytic converter at a value in the neighborhood of the stoichiometric air-fuel ratio.
Integral calculation, which is designed to generate an output through integration of a difference and thus exhibits relatively low responsiveness, can eliminate a steady difference in the air-fuel ratio downstream of the catalytic converter resulting from fluctuations in the output from the upstream oxygen concentration sensor.
Proportional calculation, which is designed to generate an output in proportion to a difference and thus exhibits high responsiveness, achieves the effect of ensuring quick recovery of a temporary difference in the air-fuel ratio downstream of the catalytic converter resulting from disturbances in the air-fuel ratio upstream of the catalytic converter. The speed of recovery increases in accordance with an increase in proportional gain.
An internal combustion engine repeatedly accelerates and decelerates, and the air-fuel ratio upstream of the catalytic converter fluctuates temporarily. The catalytic is endowed with an oxygen storage capacity with a view to absorbing a temporary deviation in the air-fuel ratio from the stoichiometric air-fuel ratio. The oxygen storage capacity is created by adding a substance having such oxygen storage capacity to the catalytic. The upper limit of the oxygen storage amount in the catalytic is determined by setting the amount of the substance.
The catalytic takes in oxygen in the exhaust gas on the lean side with respect to the stoichiometric air-fuel ratio, and maintains the air-fuel ratio of the atmosphere in the catalytic converter at a value in the neighborhood of the stoichiometric air-fuel ratio until the oxygen storage amount reaches its upper limit.
The catalytic discharges the oxygen retained thereby on the rich side with respect to the stoichiometric air-fuel ratio, and maintains the air-fuel ratio of the atmosphere in the catalytic converter at a value in the neighborhood of the stoichiometric air-fuel ratio until the oxygen amount reaches it's minimum value “0” is stopped.
The capacity to absorb fluctuations in the air-fuel ratio upstream of the catalytic converter to the lean or rich side is maximized when the oxygen storage amount of the catalytic is about half of its upper limit, so a drop in the exhaust gas purification ratio of the catalytic is minimized.
Since the oxygen storage amount of this catalytic is detectable from a minor change in the air-fuel ratio downstream of the catalytic converter in the neighborhood of the stoichiometric air-fuel ratio, air-fuel ratio control based on an output from the oxygen concentration sensor provided downstream of the catalytic converter is performed to ensure that the oxygen storage amount is maintained in a suitable state, namely, at about half of its upper limit (e.g., see JP 06-42387 A).
The invention in the conventional art is designed as described above and therefore has the following problems. The oxygen storage amount rises to its upper limit to be saturated, and the capacity to absorb disturbances to the lean side is minimized when the air-fuel ratio upstream of the catalytic converter becomes considerably lean and the mixture gas becomes almost as thin as the atmosphere due to the fuel cut. Therefore, after the fuel cut, it is desirable to recover the oxygen storage amount to the suitable state, that is, about half of its upper limit as soon as possible and restore the capacity to absorb disturbances in the air-fuel ratio.
In this case, the speed of recovery can be increased when the proportional gain in air-fuel ratio control based on the air-fuel ratio output from the oxygen concentration sensor provided downstream of the catalytic converter in PI (Proportion, Integral) control is set large.
However, if the proportional gain is set large, the amount of fuel supplied to the internal combustion engine significantly changes due to a change in the output from the oxygen concentration sensor provided downstream of the catalytic converter during normal operation in which the fuel supply is not cut, thus causing excessive torque fluctuations to deteriorate the marketability of the internal combustion engine to be controlled. As a result, there arises a problem in that the proportional gain cannot be set large.